This invention relates to an AC adapter and a method of charging a secondary battery using the AC adapter.
An AC adapter of the type described is used for charging a secondary battery which is included in or mounted on a main body such as a portable telephone set. The secondary battery may be a lithium ion cell. The AC adapter is connected to a main body through a cable having a resistor. The AC adapter has a cathode and an anode and generates an adapter voltage therebetween.
On the other hand, the main body comprises a check diode, a charge control element such as a transistor, a secondary battery, and a charge control circuit. The charge control circuit controls a charging of the secondary battery by controlling the charge control element. The charge control circuit comprises a regulator therein. The secondary battery generates a battery voltage (a charging voltage). The AC adapter has a V-I characteristic which is a constant current/constant voltage characteristic.
It will be assumed that a charging control is made to the secondary battery of the main body. Attention will be directed to a constant voltage. In this event, the charging voltage has poor precision. This is because there are a cable loss, a loss due to a contact resistor, Vf of the check diode, and so on between the AC adapter and the secondary battery. For this purpose, the charge control circuit comprises the regulator in the manner which is described above.
The adapter voltage of the AC adapter is on the high side so as to supply a voltage enable to charge although factors generating the above-mentioned voltage loss varies widely.
In such as AC adapter, it is necessary to electrically isolate and separate the primary side circuit from the secondary side circuit in order to prevent an accident such as an electric shock. As means for electrically isolating and separating, a photocoupler or an isolation transformer is generally used. In addition, it is necessary for the AC adapter to carry out a constant current control and a constant voltage control. For this purpose, it is necessary to feed a variation of a current flowing the secondary side circuit as a constant current control signal and a variation of a secondary output voltage as a constant voltage control signal back to the primary side circuit. Under the circumstances, the constant current control signal and the constant voltage control signal are returned (fed) from the secondary side circuit back to the primary side circuit through a photocoupler.
In the manner which will later be described in conjunction with FIG. 3, a conventional AC adapter circuit will be described in order to facilitate an understanding of the present invention. The AC adapter includes, as a primary side circuit, a primary rectifying and smoothing circuit, a primary winding of a transformer, a switching control circuit, and a switching element.
Supplied from an AC power supply, an input AC voltage is rectified and smoothed by the primary rectifying and smoothing circuit to convert it into an input DC voltage. The input DC voltage is applied to the primary winding of the transformer to turn the input DC voltage on and off by the switching element. Turning on and off of the switching element is controlled by an on-off control signal supplied from the switching control circuit.
In addition, the illustrated AC adapter includes, as a secondary side circuit, a secondary winding of the transformer and a secondary rectifying and smoothing circuit. Induced in the secondary winding of the transformer, an AC voltage is rectified and smoothed by the secondary rectifying and smoothing circuit to produce an adapter voltage.
The secondary side circuit comprises a constant voltage control circuit, a constant current control circuit, and a reference voltage generating circuit. The constant voltage control circuit detects a variation of the adapter voltage to produce a constant voltage control signal. The constant voltage control signal is fed back to the switching control circuit disposed in the primary side circuit as a feedback signal through an OR gate and a photocoupler. The constant current control circuit detects a current flowing in the secondary side circuit to produce a constant current control signal. The constant current control signal is also fed back to the switching control circuit disposed in the primary side circuit as the feedback signal through the OR gate and the photocoupler. The reference voltage generating circuit is for supplying a reference voltage to the constant voltage control circuit and to the constant current control circuit.
The anode is connected to ends of first and second resistors. Another end of the first resistor and another end of the second resistor are connected to the constant current control circuit. Between the cathode and the other end of the second resistor, third and fourth resistors for dividing the adapter voltage are connected. The reference voltage generating circuit is connected to the cathode. Between the reference voltage generating circuit and the other end of the second resistor, fifth and sixth resistors for dividing the reference voltage are connected. From a connection point between the fifth and the sixth resistors, a divided voltage of the reference voltage is supplied to the constant current control circuit.
On the transformer, an auxiliary winding is wound. The auxiliary winging has an end connected to the switching element, the primary rectifying and smoothing circuit, and the switching control circuit. The auxiliary winding NB has another end connected to the switching control circuit and a collector of a phototransistor of the photocoupler.
At any rate, the conventional AC adapter carries out a constant voltage control for using a fixed reference voltage.
While the battery voltage is low, the secondary battery is charged at a constant charging current. When the battery voltage reaches a predetermined voltage, a constant voltage charging is carried out. The adapter voltage is always higher than the battery voltage.
The conventional AC adapter is disadvantageous in that it generates a high voltage difference between the adapter voltage and the battery voltage in the constant voltage charging region and it results in generating heat in the charging control transistor (the charging control element) within the main body.